Since previous studies in this project have emphasized the effects of the lyophilic groups of lecithins and sterols when interacted in mixed monolayers, we have now looked at the interaction of the hydrophilic or polar groups of PE and lecithin and also those of sterols in this system. We intend to devote some effort to in vitro studies, using rat liver microsomes, on the incorporation of various diglyceride precursors into both the PE and the PC. Biosynthesis of lecithins labeled in the fatty acids or glycerol moieties will be carried out and, after isolation of specific diglycerides, they will be used as substrates to investigate relative importance of diglyceride recycling as compared with deacylation-acylation reactions. Where possible, dietary modification of the phospholipids will be induced to get a variety of species to work with. As time permits, the influence of methionine and choline on the synthesis of plasma and liver lecithin species will be conducted to see how important sequential methylation of PE is to the rat.